NAME

magic - file command’s magic number file

DESCRIPTION

This manual page documents the format of the magic file as used by the
file(1) command, version 4.21. The file(1) command identifies the type
of a file using, among other tests, a test for whether the file begins
with a certain “magic number”. The file /usr/share/file/magic specifies
what magic numbers are to be tested for, what message to print if a
particular magic number is found, and additional information to extract
from the file.
Each line of the file specifies a test to be performed. A test compares
the data starting at a particular offset in the file with a 1-byte,
2-byte, or 4-byte numeric value or a string. If the test succeeds, a
message is printed. The line consists of the following fields:
offset A number specifying the offset, in bytes, into the file of the
data which is to be tested.
type The type of the data to be tested. The possible values are:
byte A one-byte value.
short A two-byte value (on most systems) in this machine’s
native byte order.
long A four-byte value (on most systems) in this
machine’s native byte order.
quad An eight-byte value (on most systems) in this
machine’s native byte order.
string A string of bytes. The string type specification
can be optionally followed by /[Bbc]*. The “B” flag
compacts whitespace in the target, which must
contain at least one whitespace character. If the
magic has n consecutive blanks, the target needs at
least n consecutive blanks to match. The “b” flag
treats every blank in the target as an optional
blank. Finally the “c” flag, specifies case
insensitive matching: lowercase characters in the
magic match both lower and upper case characters in
the target, whereas upper case characters in the
magic, only much uppercase characters in the target.
pstring A pascal style string where the first byte is
interpreted as the an unsigned length. The string
is not NUL terminated.
date A four-byte value interpreted as a UNIX date.
qdate A eight-byte value interpreted as a UNIX date.
ldate A four-byte value interpreted as a UNIX-style date,
but interpreted as local time rather than UTC.
qldate An eight-byte value interpreted as a UNIX-style
date, but interpreted as local time rather than UTC.
beshort A two-byte value (on most systems) in big-endian
byte order.
belong A four-byte value (on most systems) in big-endian
byte order.
bequad An eight-byte value (on most systems) in big-endian
byte order.
bedate A four-byte value (on most systems) in big-endian
byte order, interpreted as a Unix date.
beqdate An eight-byte value (on most systems) in big-endian
byte order, interpreted as a Unix date.
beldate A four-byte value (on most systems) in big-endian
byte order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
beqldate An eight-byte value (on most systems) in big-endian
byte order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
bestring16 A two-byte unicode (UCS16) string in big-endian byte
order.
leshort A two-byte value (on most systems) in little-endian
byte order.
lelong A four-byte value (on most systems) in little-endian
byte order.
lequad An eight-byte value (on most systems) in little-
endian byte order.
ledate A four-byte value (on most systems) in little-endian
byte order, interpreted as a UNIX date.
leqdate An eight-byte value (on most systems) in little-
endian byte order, interpreted as a UNIX date.
leldate A four-byte value (on most systems) in little-endian
byte order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
leqldate An eight-byte value (on most systems) in little-
endian byte order, interpreted as a UNIX-style date,
but interpreted as local time rather than UTC.
lestring16 A two-byte unicode (UCS16) string in little-endian
byte order.
melong A four-byte value (on most systems) in middle-endian
(PDP-11) byte order.
medate A four-byte value (on most systems) in middle-endian
(PDP-11) byte order, interpreted as a UNIX date.
meldate A four-byte value (on most systems) in middle-endian
(PDP-11) byte order, interpreted as a UNIX-style
date, but interpreted as local time rather than UTC.
regex A regular expression match in extended POSIX regular
expression syntax (much like egrep). The type
specification can be optionally followed by /[cse]*.
The “c” flag makes the match case insensitive, while
the “s” or “e” flags update the offset to the
starting or ending offsets of the match (only one
should be used). By default, regex does not update
the offset. The regular expression is always tested
against the first N lines, where N is the given
offset, thus it is only useful for (single-byte
encoded) text. ^ and $ will match the beginning and
end of individual lines, respectively, not beginning
and end of file.
search A literal string search starting at the given
offset. It must be followed by <number> which
specifies how many matches shall be attempted (the
range). This is suitable for searching larger
binary expressions with variable offsets, using \
escapes for special characters.
default This is intended to be used with the text x (which
is always true) and a message that is to be used if
there are no other matches.
The numeric types may optionally be followed by & and a numeric value, to
specify that the value is to be AND’ed with the numeric value before any
comparisons are done. Prepending a u to the type indicates that ordered
comparisons should be unsigned.
test The value to be compared with the value from the file. If the
type is numeric, this value is specified in C form; if it is a
string, it is specified as a C string with the usual escapes
permitted (e.g. \n for new-line).
Numeric values may be preceded by a character indicating the
operation to be performed. It may be =, to specify that the
value from the file must equal the specified value, <, to
specify that the value from the file must be less than the
specified value, >, to specify that the value from the file must
be greater than the specified value, &, to specify that the
value from the file must have set all of the bits that are set
in the specified value, ^, to specify that the value from the
file must have clear any of the bits that are set in the
specified value, or ~, the value specified after is negated
before tested. x, to specify that any value will match. If the
character is omitted, it is assumed to be =. For all tests
except string and regex, operation ! specifies that the line
matches if the test does not succeed.
Numeric values are specified in C form; e.g. 13 is decimal, 013
is octal, and 0x13 is hexadecimal.
For string values, the byte string from the file must match the
specified byte string. The operators =, < and > (but not &) can
be applied to strings. The length used for matching is that of
the string argument in the magic file. This means that a line
can match any string, and then presumably print that string, by
doing >\0 (because all strings are greater than the null
string).
The special test x always evaluates to true. message The
message to be printed if the comparison succeeds. If the string
contains a printf(3) format specification, the value from the
file (with any specified masking performed) is printed using the
message as the format string. If the string begins with ‘‘\b’’,
the message printed is the remainder of the string with no
whitespace added before it: multiple matches are normally
separated by a single space.
Some file formats contain additional information which is to be printed
along with the file type or need additional tests to determine the true
file type. These additional tests are introduced by one or more >
characters preceding the offset. The number of > on the line indicates
the level of the test; a line with no > at the beginning is considered to
be at level 0. Tests are arranged in a tree-like hierarchy: If a the
test on a line at level n succeeds, all following tests at level n+1 are
performed, and the messages printed if the tests succeed, untile a line
with level n (or less) appears. For more complex files, one can use
empty messages to get just the "if/then" effect, in the following way:
0 string MZ
>0x18 leshort <0x40 MS-DOS executable
>0x18 leshort >0x3f extended PC executable (e.g., MS Windows)
Offsets do not need to be constant, but can also be read from the file
being examined. If the first character following the last > is a ( then
the string after the parenthesis is interpreted as an indirect offset.
That means that the number after the parenthesis is used as an offset in
the file. The value at that offset is read, and is used again as an
offset in the file. Indirect offsets are of the form: ((x[.[bslBSL]][+-][y]). The value of x is used as an offset in the file.
A byte, short or long is read at that offset depending on the [bslBSLm]
type specifier. The capitalized types interpret the number as a big
endian value, whereas the small letter versions interpret the number as a
little endian value; the m type interprets the number as a middle endian
(PDP-11) value. To that number the value of y is added and the result is
used as an offset in the file. The default type if one is not specified
is long.
That way variable length structures can be examined:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
>0x18 leshort <0x40 MZ executable (MS-DOS)
# skip the whole block below if it is not an extended executable
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
>>(0x3c.l) string LX\0\0 LX executable (OS/2)
This strategy of examining has one drawback: You must make sure that you
eventually print something, or users may get empty output (like, when
there is neither PE\0\0 nor LE\0\0 in the above example)
If this indirect offset cannot be used as-is, there are simple
calculations possible: appending [+-*/%&|^]<number> inside parentheses
allows one to modify the value read from the file before it is used as an
offset:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
# sometimes, the value at 0x18 is less that 0x40 but there’s still an
# extended executable, simply appended to the file
>0x18 leshort <0x40
>>(4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP)
>>(4.s*512) leshort !0x014c MZ executable (MS-DOS)
Sometimes you do not know the exact offset as this depends on the length
or position (when indirection was used before) of preceding fields. You
can specify an offset relative to the end of the last up-level field
using ‘&’ as a prefix to the offset:
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
# immediately following the PE signature is the CPU type
>>>&0 leshort 0x14c for Intel 80386
>>>&0 leshort 0x184 for DEC Alpha
Indirect and relative offsets can be combined:
0 string MZ
>0x18 leshort <0x40
>>(4.s*512) leshort !0x014c MZ executable (MS-DOS)
# if it’s not COFF, go back 512 bytes and add the offset taken
# from byte 2/3, which is yet another way of finding the start
# of the extended executable
>>>&(2.s-514) string LE LE executable (MS Windows VxD driver)
Or the other way around:
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string LE\0\0 LE executable (MS-Windows)
# at offset 0x80 (-4, since relative offsets start at the end
# of the up-level match) inside the LE header, we find the absolute
# offset to the code area, where we look for a specific signature
>>>(&0x7c.l+0x26) string UPX \b, UPX compressed
Or even both!
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string LE\0\0 LE executable (MS-Windows)
# at offset 0x58 inside the LE header, we find the relative offset
# to a data area where we look for a specific signature
>>>&(&0x54.l-3) string UNACE \b, ACE self-extracting archive
Finally, if you have to deal with offset/length pairs in your file, even
the second value in a parenthesized expression can be taken from the file
itself, using another set of parentheses. Note that this additional
indirect offset is always relative to the start of the main indirect
offset.
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
# search for the PE section called ".idata"...
>>>&0xf4 search/0x140 .idata
# ...and go to the end of it, calculated from start+length;
# these are located 14 and 10 bytes after the section name
>>>>(&0xe.l+(-4)) string PK\3\4 \b, ZIP self-extracting archive

SEEALSO

BUGS

The formats long, belong, lelong, melong, short, beshort, leshort, date,
bedate, medate, ledate, beldate, leldate, and meldate are system-
dependent; perhaps they should be specified as a number of bytes (2B, 4B,
etc), since the files being recognized typically come from a system on
which the lengths are invariant.